Aluminium die cast parts have achieved a particular significance in the automobile industry. The increasing mechanical demands placed on aluminium die cast parts in the automobile industry, prompted mainly by the substitution of steel components by aluminium alloys with the purpose of weight reduction, are met by using special AlSiMg or AlMgSi die cast alloys and a heat treatment following the casting process.
As an example, from AT 407 533 an aluminium alloy comprising >3.0 to 7.0% by weight magnesium, 1.0 to 3.0% by weight silicon, 0.3 to 0.49% by weight manganese, 0.1 to 0.3% by weight chromium, 0 to 0.15% by weight titanium, max. 0.15% by weight iron and max. 0.00005% by weight calcium and sodium each and max. 0.0002% by weight phosphorus.
In EP-B-0 792 380 an alloy is described, that comprises 3.0 to 6.0, preferably 4.6 to 5.8% by weight magnesium, 1.4 to 3.5, preferably 2.0 to 2.8% by weight silicon, 0.5 to 2.0, preferably 0.6 to 1.5% by weight manganese, max. 0.2, preferably 0.1 to 0.2% by weight titanium and max. 0.15, preferably max. 0.1% by weight iron and is already present in the rheo-structural state.
These known AlMgSi alloys are intended to be used for die casting and related processes. They have already in the cast state strength and elongation values similar to those of AlSiMg alloys, for example the known alloy of the AlSi7Mg0,3 type in the fully-hardened state (that is designated by “T6”). An important disadvantage of these AlMgSi type alloys is, however, that the 0.2% elongation limit is lower than that of AlSiMg alloys.
The 0.2% elongation limit characterises the transition from elastic to plastic deformation of a cast part and is particularly relevant in conjunction with possibly crash-affected structural parts in the automobile industry.
There are reports in the literature reports about the possibility of a short heat treatment, lasting for max. 2 hours, for the purpose of increasing the 0.2% elongation limit.
A heat treatment of die cast parts produced from the above mentioned AlMgSi alloys has, however, numerous disadvantages. First of all the cost advantage, that can be achieved by such alloys, is annulled. Further basic disadvantages of the heat treatment are typical defects of the die cast parts, like distortion, and above all blisters, occurring due to the thermal disintegration of the mould release agents included, known under the term “blister”. A distortion, however, negates the advantage of the process of die cast parts, namely production with dimensions close to the final ones.
In the case of die cast parts, that are not subjected to heat treatment to increase the 0.2% elongation limit, the scope of application of the above described aluminium alloys is limited due to the relatively low 0.2% elongation limit, since especially for die cast parts subjected to load greater strength properties are required. The application of die cast parts, produced from such alloys, can be achieved only by increasing the thickness of their wall. However, the increase of the wall thickness reduces or negates the weight advantage achievable by using aluminium.